1. Field of the Invention
The present invention relates to a nitride semiconductor substrate comprising a group 13 nitride and suitably used for a semiconductor power device.
2. Description of the Related Art
Group 13 nitride semiconductors including GaN attract attention as next-generation semiconductor power device materials. In particular, a nitride semiconductor substrate having grown a group 13 nitride semiconductor crystal epitaxially on a substrate of different materials, such as sapphire, silicon, etc., has many advantages in respect of the balance between a device property and manufacturing cost. Especially, one that uses a silicon substrate which allows a diameter of 6 inches is very useful industrially.
Incidentally, it is known that a phenomenon called current collapse (also called a current slump) arises in a nitride semiconductor power device. “Current collapse” is a phenomenon where when current is switched from OFF to ON, it acts as if resistance of a channel increased, and conduction current decreased.
It is generally said that the cause of current collapse relates to electron trapping in the defects at the outermost surface of the device or the defects in the buffer layer. As a measure for improving such current collapse, outermost surface passivation by a SiNx film etc., a field plate structure for reducing intensive electric field, etc. are known (for example, see Japanese Patent Application Publication No. 2005-286135 (Patent Literature 1) and a Japanese Patent Application Publication No. 2009-252756 (Patent Literature 2)).
Further, a deep level defect formed by residual carbon in this buffer layer is known as a carrier trap site in the buffer layer relevant to current collapse. On the other hand, residual carbon compensates for residual donor in a GaN-type semiconductor crystal, and allows this crystal to have high resistance. Thus, how to form a buffer layer caused to have high resistance by doping has also been examined. For example, Japanese Patent Application Publication No. 2007-251144 (Patent Literature 3) discloses the formation of the buffer layer including the nitride semiconductor layer doped with carbon within a range which does not adversely affect the current collapse.
Furthermore, Japanese Patent Application Publication No. 2012-174697 (Patent Literature 4) discloses that a nitride semiconductor layer is stacked on a substrate through a buffer layer where n-type GaN is doped with carbon, so that a position for taking a carbon atom is controlled, a carrier is compensated effectively, and the buffer layer of high resistance can be formed.
However, as disclosed in Patent Literature 3 above, it is preferable that the concentration of the above-mentioned residual carbon is lower in order to control the current collapse. On the other hand, residual carbon causes the GaN-type semiconductor crystal to have high resistance, and serves to reduce the leakage current from the device. That is to say, when controlling the residual carbon concentration in the buffer layer, there is a trade-off between the current collapse property and the leakage current reduction. Thus, it is not necessarily easy to improve the current collapse property and the leakage property and satisfy both simultaneously only by doping the buffer layer with carbon.
Furthermore, even by the method in which the buffer layer comprising Si-doped n-type GaN is doped with carbon as disclosed in Patent Literature 4 above, it is difficult to obtain the nitride semiconductor substrate which can fully satisfy the high withstand voltage property, while satisfying both the current collapse property and the leakage property.
Therefore, in order to realize the nitride semiconductor electron device with high withstand voltage by doping, there is a need for a means for controlling the current collapse by a method other than the method for controlling a carbon concentration in the buffer layer doped with carbon.